Health Risk and Exposure Assessment for 

Ozone 

Final Report 

Executive Summary 

 
 

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 EPA-452/R-14-004f 

August 2014 

Health Risk and Exposure Assessment for Ozone 

Final Report 

Executive Summary 

U.S. Environmental Protection Agency 

Office of Air and Radiation 

Office of Air Quality Planning and Standards 

Health and Environmental Impacts Division 

Risk and Benefits Group 

Research Triangle Park, North Carolina 27711 

 
 

DISCLAIMER

This final document has been prepared by staff from the Risk and Benefits Group, Health and 

Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. Environmental 

Protection Agency. Any findings and conclusions are those of the authors and do not necessarily reflect 

the views of the Agency. 

 

Questions related to this document should be addressed to Dr. Bryan Hubbell, U.S. 

Environmental Protection Agency, Office of Air Quality Planning and Standards, C539-07, Research 

Triangle Park, North Carolina 27711 (email: hubbell.bryan@epa.gov). 
 

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Executive Summary Health Risk and Exposure Assessment 

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Introduction 

s part of the review of the ozone (O3) 

National Ambient Air Quality Standards 

(NAAQS), EPA has prepared this Health 

Risk and Exposure Assessment (HREA) to 

provide estimates of exposures to O3 and 

resulting mortality and morbidity health risks. 

The health effects evaluated in this HREA 

are based on the findings of the O3 ISA (U.S. 

EPA, 2013) that short term O3 exposures are 

causally related to respiratory effects, and 

likely causally related to cardiovascular 

effects, and that long term O3 exposures are 

likely causally related to respiratory effects. 

The assessment evaluated total exposures 

and risks associated with the full range of 

observed O3 concentrations. In addition, 

the HREA estimated the incremental 

changes in exposures and risks associated 

with ambient air quality adjusted to just 

meeting the existing standard of 75 ppb 

and just meeting potential alternative 

standard levels of 70, 65, and 60 ppb using 

the form and averaging time of the existing 

standard, which is the annual 4th highest 

daily maximum 8-hour O3 concentration, 

averaged over three consecutive years. 

The results of the HREA are developed to 

inform the O3 Policy Assessment (PA) in 

considering the adequacy of the existing O3

standards, and potential risk reductions 

associated with potential alternative levels 

of the standard. For added context 

regarding existing O3 air quality and the 

potential impact to public health, initial 

nonattainment area designations have 

been made for 46 areas in the U.S. with 

ambient O3 concentrations exceeding the 

existing standard (77 FR 30160). The figure 

below provides the locations of 

nonattainment areas and their respective 

classifications and includes 227 counties 

with an estimated 2010 population of just 

over 123 million people. 

As described in the conceptual framework 

and scope in Chapters 2 and 3, 

respectively, the HREA discusses air quality 

considerations (Chapter 4) and evaluates 

exposures and lung function risk in 15 urban 

study areas (Chapters 5 and 6, respectively) 

and risks based on application of results of 

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epidemiology studies in a subset of 12 urban 

study areas (Chapter 7). In addition, to 

place the urban study area analyses in a 

broader context, the assessment estimated 

the national burden of mortality associated 

with recent O3 levels, and evaluated the 

representativeness of the urban areas in 

characterizing O3 exposures and risks across 

the U.S. (Chapter 8). To further facilitate 

interpretation of the results of the exposure 

and risk assessment, Chapter 9 provides a 

synthesis of the various results, focusing on 

comparing and contrasting those results to 

identify common patterns, or important 

differences. It also includes an overall 

integrated characterization of exposure and 

risk in the context of key policy relevant 

questions. 

Conceptual Framework 

and Scope 

he HREA provides information to answer 

key policy-relevant risk questions with 

regards to evaluation of the adequacy 

of the existing standards and evaluation of 

potential alternative standards such as: 

“To what extent do risk and/or exposure 

analyses suggest that exposures of concern 

for O3-related health effects are likely to 

occur with existing ambient levels of O3 or 

with levels that just meet the O3 standard? 

To what extent do alternative standards, 

taking together levels, averaging times and 

forms, reduce estimated exposures and risks 

of concern attributable to O3 and other 

photochemical oxidants, and what are the 

uncertainties associated with the estimated 

exposure and risk reductions?”

 

In answering these key questions, the HREA 

evaluates total exposures and risks 

associated with the full range of observed 

O3 concentrations, as well as the 

incremental changes in exposures and risks 

for just meeting the existing standard and 

just meeting several alternative standards. 

With regard to selecting alternative levels for 

the 8-hour O3 standards for evaluation in the 

quantitative risk assessment, we base the 

range of selected levels on the evaluations 

of the evidence provided in the first draft 

PA, which received support from the CASAC 

in their advisory letter on the first draft PA. 

The first draft PA recommended evaluation 

of 8-hour maximum concentrations in the 

range of 60 to 70 ppb, with possible 

consideration of levels somewhat below 60 

ppb. 

Ozone concentrations from 2006-2010 are 

used in estimating exposures and risks for the 

15 urban study areas. Because of the yearto-year variability in O3 concentrations, the 

assessment evaluates air quality scenarios 

for just meeting the existing and potential 

alternative standards based on multiple 

years of O3 data to better capture the high 

degree of variability in meteorological 

conditions, as well as reflecting years with 

higher and lower emissions of O3 precursors. 

The 15 urban study areas were selected to 

be generally representative of U.S. 

populations, geographic areas, climates, 

and varying O3 and co-pollutant levels. 

These urban study areas include Atlanta, 

GA; Baltimore, MD; Boston, MA; Chicago, IL; 

Cleveland, OH; Dallas, TX; Denver, CO; 

Detroit, MI; Houston, TX; Los Angeles, CA; 

New York, NY; Philadelphia, PA; 

Sacramento, CA; St. Louis, MO; and 

Washington, D.C. 

We have identified the following goals for 

the urban area exposure and risk 

assessments: (1) to provide estimates of the 

number and percent of people in the 

general population and in at-risk 

populations and lifestages with O3 exposures 

above health-based benchmark levels; (2) 

to provide estimates of the number and 

percent of people in the general population 

and in at-risk populations and lifestages with 

impaired lung function (defined based on 

decrements in forced expiratory volume in 

one second (FEV1) resulting from exposures 

to O3; (3) to provide estimates of the 

potential magnitude of premature mortality 

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associated with both short-term and longterm O3 exposures, and selected morbidity 

health effects associated with short-term O3

exposures; (4) to evaluate the influence of 

various inputs and assumptions on risk 

estimates to the extent possible given 

available methods and data; (5) to gain 

insights into the spatial and temporal 

distribution of risks associated with O3

concentrations just meeting existing and 

alternative standards, patterns of risk 

reduction associated with meeting 

alternative standards relative to the existing 

standard, and uncertainties in the estimates 

of risk and risk reductions. 

In working towards these goals, we follow a 

conceptual framework, shown in the figure 

above , comprised of an air quality 

characterization, a review of relevant 

scientific evidence on health effects, the 

 

1 The CMAQ model and associated 

documentation is available for download at 

https://www.cmascenter.org/cmaq/. 

2 The APEX model and associated 

documentation is available for download at 

http://www.epa.gov/ttn/fera/human_apex.html 

modeling of exposure, the modeling of risk, 

and a risk characterization. As shown in this 

framework, air quality is characterized 

primarily by the combined use of ambient 

monitoring data available in the EPA Air 

Quality System (AQS), and a spatial 

interpolation approach (Voronoi Neighbor 

Averaging, VNA), along with Higher-Order 

Decoupled Direct Method (HDDM) 

capabilities in the Community Multi-scale Air 

Quality (CMAQ)1 model. The modeling of 

personal exposure and estimation of risks, 

which rely on personal exposure estimates, 

are implemented using the EPA’s Air 

Pollution Exposure model (APEX)2. Modeling 

of population level risks for health endpoints 

based on application of results of 

epidemiological studies is implemented 

using the environmental Benefits Mapping 

and Analysis Program (BenMAP)3, a peer 

reviewed software tool for estimating risks 

3 The BenMAP model and associated 

documentation is available for download at 

http://www.epa.gov/air/benmap/
 

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and impacts associated with changes in 

ambient air quality. The overall 

characterization of risk draws from the results 

of the exposure assessment and both types 

of risk assessment. 

Air Quality Characterization 

n this analysis, we employed a 

photochemical model-based adjustment 

methodology (Simon et al., 2013) to 

estimate the change in observed hourly O3

concentrations at a given set of monitoring 

sites resulting from across-the-board 

reductions in U.S. anthropogenic NOx and/or 

VOC emissions. This information was then 

used to adjust recent O3 concentrations 

(2006-2010) in the 15 urban study areas to 

reflect just meeting the existing 8-hour O3

standard of 75 ppb and for just meeting 

potential alternative standard levels of 70, 

65, and 60 ppb. Because the form of the 

existing O3 standard is based on the 3-year 

average of the 4th highest daily maximum 8-

hour average, we simulate just meeting the 

potential alternative standards for two 

periods, 2006-2008 and 2008-2010. 

The use of the model-based adjustment 

methodology is an example of how we 

have brought improvements into this review 

that better represent current scientific 

understanding. The model-based 

adjustment methodology represents a 

substantial improvement over the quadratic 

rollback method used to adjust O3

concentrations in past reviews. For 

example, while the quadratic rollback was a 

purely mathematical technique which 

attempted to reproduce the distribution of 

observed O3 concentrations just meeting 

various standards, the new methodology 

uses photochemical modeling to simulate 

the response in O3 concentrations due to 

changes in precursor emissions based on 

current understanding of atmospheric 

chemistry and pollutant transport. Second, 

quadratic rollback used the same 

mathematical formula to adjust 

concentrations at all monitors within each 

urban study area for all hours, while modelbased adjustment methodology allows the 

adjustments to vary both spatially across 

each study area and temporally across 

hours of the day and across seasons. Finally, 

quadratic rollback was designed to only 

allow decreases in O3 concentrations, while 

the model-based adjustment methodology 

allows both increases and decreases in O3

concentrations, which more accurately 

reflects the scientific understanding that 

increases in O3 concentrations may occur in 

response to reductions in NOx emissions in 

some situations, such as in urban areas with 

a large amount of NOx emissions. 

Several general trends are evident in the 

changes in O3 patterns across the urban 

study areas and across the different 

standards under consideration. In all 15 

urban study areas, peak O3 concentrations 

tended to decrease while the lowest O3

concentrations tended to increase as the 

concentrations were adjusted to just meet 

the existing and potential alternative 

standards. In addition, high and mid-range 

O3 concentrations generally decreased in 

rural and suburban portions of the case 

study areas, while the O3 response to NOx

reductions was more varied within urban 

core areas. In particular, while the annual 

4th highest daily maximum 8-hour 

concentrations generally decreased in the 

urban core of the study areas in response to 

reductions in NOx emissions, the seasonal 

mean of the daily maximum 8-hour O3

concentrations did not change significantly, 

though it did exhibit some increases or 

decreases in the various study areas as the 

distribution of O3 was further adjusted to 

meet lower potential alternative standards. 

The adjustments to O3 to reflect just meeting 

existing and potential alternative standards 

are made by decreasing only U.S. emissions 

of anthropogenic NOx primarily, and in a 

few instance, both NOx and VOC 

reductions. As such, the estimated changes 

in exposure and risk, based on these air 

quality changes, are solely attributable to 

changes in U.S. emissions and are not meant 

to reflect a specific air emission control 

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strategy that might be used by a state or 

urban area to meet a standard. 

Human Exposure Modeling 

he population exposure assessment 

evaluated exposures to O3 using the 

APEX exposure model which uses 

time-activity diary and anthropometric 

data coupled with local meteorology, 

population demographics, and O3

concentrations to estimate the number 

and percent of study group individuals 

above exposure benchmarks. The 

analyses examined exposure to O3 for the 

general population, all school-aged 

children (ages 5-18), asthmatic schoolaged children, asthmatic adults (ages > 

18), and older persons (ages 65 and 

older), with a focus on when exposed 

individuals were engaged in moderate or 

greater exertion, for example, children 

engaged in outdoor recreational 

activities. Exposure is assessed in the 15 

urban study areas for recent O3 (2006-

2010) and for O3 adjusted to just meet 

existing and potential alternative 

standards for two averaging periods 

(2006-2008 and 2008-2010). The analysis 

provided estimates of the number and 

percent of several study groups of interest 

exposed to concentrations above three 

health-relevant 8-hour average O3

exposure benchmarks: 60, 70, and 80 ppb. 

These benchmarks were selected to 

provide perspective on the public health 

impacts of O3-related health effects that 

have been demonstrated in human 

clinical and toxicological studies, but 

cannot currently be evaluated in 

quantitative risk assessments, such as lung 

inflammation and increased airway 

responsiveness. The O3 ISA includes 

studies showing significant effects at each 

of these benchmark levels. 

The analysis found that children are the 

study group of greatest concern for O3

exposures due to the greater amount of 

afternoon time they spend outdoors 

engaged in moderate or higher exertion 

activities and that they do so more 

frequently of any of the at-risk study groups. 

Based on this, we focus on the results for 

children in this subsequent discussion. The 

two figures below show the average across 

2006-2010 of the percentage of schoolaged children experiencing 8-hour exposure 

greater than 60 ppb at least once (top) T	
 

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and at least twice (bottom) per O3 season. 

Based on this information, no more than 17 

percent of any study group in any study 

area, on average, was exposed at least 

once at or above the 60 ppb benchmark, 

when meeting the existing standard. When 

meeting a standard level of 70 ppb, less 

than 11 percent of any study group in any 

study area, on average, was exposed at 

least once at or above the 60 ppb exposure 

benchmark. Adjusting ambient O3 to just 

meet a standard level of 65 ppb is 

estimated to reduce the percent of persons 

at or above an exposure benchmark of 60 

ppb to 4 percent or less of any study group 

and study area, on average. 

For the exposure benchmark of 70 ppb, on 

average less than 4 percent of any study 

group, including all school-age children, in 

any study area, was exposed at least once 

at or above the exposure benchmark when 

meeting the existing standard (not shown). 

For the highest exposure benchmark of 80 

ppb, on average less than 1 percent of any 

study group in any study area was exposed 

at least once at or above the exposure 

benchmark when meeting the existing 

standard (not shown). As expected, with 

the lower ambient O3 levels associated with 

just meeting lower alternative standard 

levels, the percentages of at-risk study 

groups experiencing exposures above the 

benchmark levels are smaller than when just 

meeting the existing standard. 

In considering two or more exceedances of 

the 60 ppb benchmark, on average less 

than 8 percent of any study group in any 

study area experience such 8-hour 

exposures when air quality is adjusted to 

meet the existing standard. There were no 

persons estimated to experience any multiday exposures above the exposure 

benchmark of 80 ppb for any study group in 

any study area, while less than 1 percent of 

persons were estimated to experience two 

or more 8-hour exposures at or above 70 

ppb, when meeting the existing standard or 

any of the alternative standard levels (not 

shown). 

In addition, the exposure assessment also 

identified the specific microenvironments 

and activities that contribute most to 

exposure and evaluated at what times and 

for how long individuals were in key 

microenvironments and were engaged in 

key activities, with a focus on persons 

experiencing the highest daily maximum 8-

hour exposure within each study area. That 

analysis indicated that children are an 

important exposure study group, largely as a 

result of the combination of large amounts 

of afternoon time spent outdoors and their 

engagement in moderate or high exertion 

level activities. Highly exposed children, on 

average, spend half of their outdoor time 

engaged in moderate or greater exertion 

levels, such as participating in sporting 

activities. In addition, any people spending 

a large portion of their time outdoors during 

afternoon hours experienced the highest 8-

hour O3 exposure concentrations given that 

ambient O3 concentrations are typically 

highest during this time of day and other 

microenvironments evaluated, particularly 

indoor microenvironments, have much 

lower O3 concentrations than ambient 

concentrations. Simulations of highly 

exposed adults indicated that they also 

spent large amounts of afternoon time 

outdoors engaged in moderate or greater 

exertion level activities though on average, 

not participating in these events as 

frequently as children. 

Health Risks Based on 

Controlled Human Exposure 

Studies 

his analysis uses the estimated O3

exposures from APEX, combined with 

results from controlled human exposure 

studies, to estimate the number and 

percent of at-risk study groups (all children, 

children with asthma, adults aged 18-35, 

adults aged 36-55, and outdoor workers) 

experiencing selected decrements in lung 

function. The analysis focuses on estimates 

of the percent of each at-risk population 

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experiencing a reduction in lung function for 

three different health effect levels: 10, 15, 

and 20 percent decrements in FEV1. These 

health effect levels were selected based on 

the published literature and conclusions 

drawn regarding the adversity associated 

with increasing lung function decrements 

(O3 ISA, Section 6.2.1.1; Henderson, 2006). 

Lung function decrements of 10 percent 

and 15 percent in FEV1 are considered 

moderate decrements; 10 percent is 

considered potentially adverse for people 

with lung disease, while a 15 percent is 

potentially adverse for active healthy 

people. A 20 percent decrement in FEV1 is 

considered a large decrement that is 

potentially adverse for healthy people and 

can potentially cause more serious effects in 

people with lung disease. 

Two types of FEV1 risk were used to estimate 

lung function risks. One model was based 

on application of a population level 

exposure-response (E-R) function consistent 

with the approach used in the previous O3

review. The second model was based on 

application of an individual level risk 

function (the McDonnell-Stewart-Smith 

(MSS) model), newly introduced for this 

review. The main difference between the 

two models is that the MSS model includes 

responses associated with a wider range of 

exposure protocols used in the original 

controlled human exposure studies (i.e., 

variable levels of exertion, lengths of 

exposures, and patterns of exposure 

concentrations) than compared to the 

exposure-response model of previous 

reviews. The models are similar in that both 

models have a logistic form and are less 

sensitive to changes at very low 

concentrations of O3 than to higher O3

concentrations. As a result, the models 

estimate very few FEV1 responses > 10% 

when ambient concentrations are below 

20 ppb and very few FEV1 responses > 15% 

when ambient concentrations are below 

40 ppb. Because the individual level E-R 

function approach allows for a more 

complete estimate of risk, we focus on the 

results of the MSS model for this discussion. 

Lung function risks were estimated for each 

of the 15 urban case study areas for recent 

air quality conditions (2006-2010) and for air 

quality adjusted to just meet the existing 

and alternative standards for two design 

value periods (2006-2008 and 2008-2010). As 

with the exposure assessment, we focus on 

lung function decrements in children as they 

are the study group likely to have the 

greatest percentage of that group at risk 

due to higher levels of exposure and greater 

levels of exertion. The figure below shows 

the lung function risks associated with just 

meeting the existing and potential 

alternative standard levels, where risk is 

taken to be the average value for each 

study area (across all years considered) of 

the percent of school-aged children with 

FEV1 decrement of 10 percent or greater. 

This figure shows that there are decreases in 

incremental risk for all 15 cities in the 

progression from the level of the existing 

standard, 75 ppb to the alternative 

standard levels of 70, 65, and 60 ppb. The 

estimated risks in this figure for Washington, 

DC, for example, are about 16 percent for 
 

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the existing standard level of 75 ppb and 

about13 percent for the alternative 

standard level of 70 ppb. The length of the 

brown bar is the incremental reduction (3 

percent) in the percent of persons at risk, 

when adjusting air quality from the existing 

standard of 75 ppb to the 70 ppb 

alternative standard. The pattern of 

reductions for lung function decrements 

larger than 15 and 20 percent are similar to 

those illustrated here (not shown). 

Health Risks Based on 

Application of Results of 

Epidemiological Studies 

he epidemiology-based risk assessment 

evaluated mortality and morbidity risks 

from short-term exposures, as well as 

mortality risks from long-term exposures to 

O3, by applying concentration-response (CR) functions derived from epidemiology 

studies. Most of the endpoints evaluated in 

epidemiology studies are for the entire study 

population. Because most mortality and 

hospitalizations occur in older persons, the 

risk estimates for this portion of the analysis 

are thus more focused in adults rather than 

children, and thus differ in focus compared 

to the human exposure and lung function 

risk assessments. The analysis included both 

a set of urban area study areas and a 

national-scale assessment. 

The urban study area analyses evaluated 

mortality and morbidity risks, including 

emergency department (ED) visits, hospital 

admissions (HA), and respiratory symptoms 

associated with recent O3 concentrations 

(2006-2010) and with O3 concentrations 

adjusted to just meet the existing and 

alternative O3 standards. Mortality and 

hospital admissions were evaluated in 12 

urban study areas (a subset of the 15 urban 

study areas evaluated in the exposure and 

lung function risk assessments), while ED visits 

and respiratory symptoms were evaluated in 

a subset of study areas with supporting 

epidemiology studies. The 12 urban areas 

were: Atlanta, GA; Baltimore, MD; Boston, 

MA; Cleveland, OH; Denver, CO; Detroit, MI; 

Houston, TX; Los Angeles, CA; New York, NY; 

Philadelphia, PA; Sacramento, CA; and St. 

Louis, MO. The urban study analyses focus 

on risk estimates for the middle year of each 

three-year ambient standard simulation 

period (2006-2008 and 2008-2010) in order to 

provide estimates of risk for a year with 

generally higher O3 levels (2007) and a year 

with generally lower O3 levels (2009). 

 

In previous O3 NAAQS reviews, health risks 

were estimated for the portion of total O3

attributable to North American 

anthropogenic sources (referred to in 

previous O3 reviews as “policy relevant 

background”). In contrast, this assessment 

provides risk estimates for the urban study 

areas for O3 concentrations down to zero, 

reflecting the lack of evidence for a 

detectable threshold in the C-R functions 

(O3 ISA), and the understanding that U.S. 

populations may experience health risks 

associated with O3 resulting from emissions 

from all sources, both natural and 

anthropogenic, and within and outside the 

U.S. 

The figure below shows the results of the 

mortality (top panel) and respiratory hospital 

admissions (bottom panel) risk assessments 

for all 12 urban areas associated with shortterm exposure to O3, showing the effect on 

the incidence per 100,000 population just 

meeting the existing 75 ppb standard and 

potential alternative O3 standards of 70, 65, 

and 60 ppb in 2007. The overall trend across 

urban areas is relatively small decreases in 

mortality and morbidity risk as air quality is 

adjusted to just meet incrementally lower 

standard levels. In New York, there are 

somewhat greater decreases, reflecting the 

relatively large emission reductions used to 

adjust air quality to just meet the 65 ppb 

alternative standard, and the substantial 

change in the distribution of O3

concentrations that resulted. Risks vary 

substantially across urban study areas; 

however, the general pattern of risk 

reductions associated with air quality 

adjusted to just meet alternative standards is 

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similar between urban study areas. Risks are 

generally lower in 2009 (not shown) relative 

to 2007; though the patterns of reductions 

are very similar between the two years. On 

average, compared with air quality 

adjusted to just meet the existing standard, 

mortality and respiratory hospitalization risks 

decrease by 5% or less for where ambient 

concentrations are adjusted to meet a 

standard level of 70 ppb, 10% or less for 

meeting a level of 65 ppb, and 15% or less 

for meeting a level of 60 ppb. Larger risk 

reductions are estimated on days with 

higher O3. 

 We also evaluated mortality risks in the 12 

urban study areas associated with long-term 

O3 exposures (based on the April to 

September average of the daily maximum 

one-hour ambient O3 concentrations). The 

figure below shows the results of the longterm mortality risk assessment for all 12 urban 

study areas, showing the effect on the 

incidence per 100,000 population 

considering air quality adjusted to 

just meeting the existing standard 

and potential alternative O3

standard levels of 70, 65, and 60 

ppb in 2007. Risks from long-term 

exposures after adjusting air 

quality to just meet the existing 

standard are substantially greater 

than risks from short-term 

exposures, ranging from 15 to 30 

percent of respiratory mortality 

across urban areas. However, the 

percent reductions in risks are 

similar to those for mortality from 

short-term exposures, e.g., less 

than 10 percent reduction in risk 

relative to just meeting the existing 

standard in most areas when just 

meeting the 70 ppb and 65 ppb 

alternative standards, and less 

than 20 percent reductions when 

just meeting the 60 ppb 

alternative standard level. 
 

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Mortality and morbidity risks generally do not 

show large responses to meeting existing or 

alternative levels of the standard for several 

reasons. First, these risks are based on C-R 

functions that are approximately linear 

along the full range of ambient O3

concentrations, and therefore reflect the 

impact of changes in O3 along the 

complete range of 8-hour average O3

concentrations. This includes days with low 

baseline O3 concentrations that are 

predicted to have increases in O3

concentrations, as well as days with higher 

starting O3 concentrations that are 

predicted to have decreases in O3

concentrations when adjusting air quality to 

just meet the existing and potential 

alternative standards. Second, these risks 

reflect changes in the urban area-wide 

monitor average, that will not be as 

responsive to air quality adjustments as the 

design value monitor, and includes monitors 

with both decreases and increases in 8-hour 

concentrations. Third, the days and 

locations with predicted increases in O3

concentrations (generally those with low- to 

mid-range starting O3 concentrations) 

resulting from adjusting air quality to just 

meeting the existing or alternative 

standard levels generally are 

frequent enough to offset days and 

locations with predicted decreases 

in O3. The focus of the 

epidemiological studies on urban 

area-wide average O3

concentrations, and the lack of 

threshold levels coupled with the 

linear nature of the C-R functions 

mean that in this analysis, the 

impact of a peak-based standard 

(i.e., designed to reduce peak 

concentrations regardless of effects 

on low or mean concentrations) on 

estimates of mortality and morbidity 

risks based on results of those studies 

is relatively small. However, we are 

not able to draw strong conclusions 

about the results across urban study 

areas, because of the limited 

number of urban study areas 

represented for most of the 

endpoints. 

The national-scale epidemiology-based risk 

assessment evaluated only mortality 

associated with recent O3 concentrations 

across the entire U.S for 2006-2008. The 

national-scale assessment is a complement 

to the urban-scale analysis, providing both a 

broader assessment of O3-related health 

risks across the U.S. It demonstrates that 

there are O3 risks across the U.S, not just in 

urban areas, even though the O3 levels in 

many areas were lower than the existing 

standard level. We estimated 15,000 

premature O3-related non-accidental 

deaths (all ages) annually associated with 

short-term exposure to recent O3 levels 

across the continental U.S. for 2007, MaySeptember. For long-term mortality, we 

estimated 45,000 premature O3-related 

adult (age 30 and older) respiratory deaths 

annually for 2007, April-September. While 

we did not assess the changes in risk at a 

national level associated with just meeting 

existing and potential alternative standards, 

just meeting existing and potential 

alternative standards would likely reduce O3

concentrations both in areas that are not 

meeting those standards and in locations 
 

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surrounding those areas, leading to risk 

reductions that are not captured by the 

urban-scale analysis.

Representativeness of 

Exposure and Risk Results 

and Associated 

Uncertainties 

s part of this assessment, we conducted 

several analyses to determine the 

extent to which our selected urban 

areas represent: (1) the highest mortality 

and morbidity risk areas in the U.S.; and (2) 

the types of patterns of O3 air quality 

changes that we estimate would be 

experienced by the overall U.S. population 

in response to emissions reductions that 

would decrease peak O3 concentrations to 

meet the existing standard or lower 

alternative O3 standard levels. 

We selected urban study areas for the 

exposure and risk analyses based on criteria 

that included O3 levels, at-risk study groups, 

and related factors that were designed to 

ensure we captured areas and persons likely 

to experience high O3 exposures and risks. 

Based on the comparisons of distributions of 

risk characteristics, the selected urban study 

areas represent urban areas that are 

among the most populated in the U.S., have 

relatively high peak O3 levels, and capture a 

wide range of city-specific mortality risk 

effect estimates. The analyses found that 

the O3 mortality risk for short-term O3

exposures in the 12 urban study areas are 

representative of the full distribution of U.S. 

O3-related mortality, representing both high 

end and low end risk counties. For the longterm exposure related mortality risk metric, 

the 12 urban study areas are representative 

of the central portion of the distribution of 

risks across all U.S. counties, however, the 

selected 12 urban areas do not capture the 

very highest (greater than 98th percentile) or 

lowest (less than 25th percentile) ends of the 

national distribution of long-term exposurerelated O3-related risk. 

While we selected urban study areas to 

represent those populations likely to 

experience elevated risks from O3 exposure, 

we did not include amongst the selection 

criteria the responsiveness of O3 in the urban 

study area to decreases in O3 precursor 

emissions that would be needed to just 

meet existing or potential alternative 

standards. The additional analyses we 

conducted suggest that many of the urban 

study areas may show O3 responses that are 

typical of other large urban areas in the U.S., 

but may not represent the response of O3 in 

other populated areas of the U.S. These 

other areas, including suburban areas, 

smaller urban areas, and rural areas, would 

be more likely than our urban study areas to 

experience area-wide average decreases 

in mean O3 concentrations and, therefore, 

associated decreases in mortality and 

morbidity risks, as O3 standards are met. 

Even though large urban areas have high 

population density, the majority of the U.S. 

population lives outside of these types of 

urban core areas, and thus, a large 

proportion of the population is likely to 

experience greater mortality and morbidity 

risk reductions in response to reductions in 8-

hour O3 concentrations than are predicted 

by our modeling in the 12 selected urban 

case study areas. 

Because our selection strategy for risk 

modeling was focused on identifying areas 

with high risk, we tended to select large 

urban population centers. This strategy was 

largely successful in including urban areas in 

the upper end of the O3 risk distribution. 

However, this also led to an 

overrepresentation of the populations living 

in locations where we estimate increasing 

mean seasonal O3 would occur in response 

to decreases in O3 precursor emissions that 

would be needed to just meet existing or 

alternative standards. The implication of this 

is that our estimates of mortality and 

morbidity risk reductions for the selected 

urban areas should not be seen as 

representative of potential risk reductions for 

most of the U.S. population, and are likely to 

understate the average risk reduction that 

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Executive Summary Health Risk and Exposure Assessment 

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would be experienced across the 

population. 

While the best available science 

information and methodologies are 

used to estimate exposures and 

associated health risk, there remain 

significant uncertainties in each of the four 

primary analytical areas of this HREA. For 

example, a number of important 

uncertainties are identified regarding the 

modeling approaches used to characterize 

air quality (i.e., the CMAQ modeling, the 

HDDM method used simulate alternative air 

quality scenarios, application of 2007 

modeled sensitivities for months and years 

not modeled), though results of our 

uncertainty characterization show limited 

instances of the potential for either under- or 

over-estimating ambient concentrations 

while also having a limited range of 

potential bias, generally less than a few 

ppb. Similar conclusions are drawn 

regarding the most important uncertainties 

in estimating exposures, in particular those 

concerning concentrations at or above 

exposure benchmark levels (i.e., the human 

activity pattern data and afternoon time 

spent outdoors). Extensive activity pattern 

data evaluations considering numerous 

influential factors (e.g., survey year, 

geographic region, health condition) 

combined with confidence in the 

characterization of air quality used as input 

to exposure calculations suggests a limited 

potential for bias in our exposure estimates. 

When considering the FEV1 risk estimates, 

the most important uncertainties are found 

in the lung function risk model itself and the 

moderate to high sensitivity of FEV1

responses to changes in values used for 

certain input variables (i.e., inter- and intraindividual variability in response). Important 

uncertainties in our epidemiological-based 

risk estimates are associated with the C-R 

functions (i.e., overall shape of function at 

low O3 concentrations and exposure 

measurement error) and its application (i.e., 

the urban study area risk modeling domains 

are extended beyond the original urban 

area from which the functions were 

derived). 

Synthesis 

o facilitate interpretation of the results of the 

exposure and risk assessment, this 

assessment provides a synthesis of the 

various results, focusing on comparing and 

contrasting those results to identify common 

patterns, or important differences. 

Consistent with the available evidence, we 

estimated exposures relative to several 

health-based exposure benchmarks, lung 

function risks based on a threshold 

exposure-response model of lung function 

decrements, and mortality and morbidity 

risks based on non-threshold C-R functions. 

These three different analyses result in 

differing sensitivities of results to changes in 

ambient O3 concentrations. Because the 

three metrics are affected differently by 

changes in O3 at low concentration levels, it 

is important to understand these changes in 

O3 at low concentrations in interpreting 

differences in the results across metrics. 

The exposure benchmark analysis is the least 

sensitive to changes in O3 in the lower part 

of the O3 concentration distribution, 

because the lowest exposure benchmarks is 

at 60 ppb, a level above the portion of the 

overall O3 concentrations distribution where 

we observed increases when adjusting air 

quality to just meet the existing and 

alternative standards. Because the 

modeled exposures will always be less than 

or equal to the ambient monitor 

concentrations, a benchmark of exposure 

at 60 ppb is above the range of O3

concentrations where the model-based 

adjustment approach estimates increases in 

concentrations. Thus, this risk metric would 

most reflect the decreases in O3 at high 

concentrations that are expected to result 

from adjusting air quality to just meeting the 

existing and potential alternative standards. 

The lung function risk analysis is less sensitive 

than the mortality and morbidity risk 

assessments to changes at very low 

concentrations of O3, because the risk 

function is logistic and shows little response 

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Executive Summary Health Risk and Exposure Assessment 

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at lower O3 doses that tend to occur when 

ambient concentrations are lower 

(generally less than 20 ppb for the 10 

percent FEV1 decrement and generally less 

than 40 ppb for the 15 percent FEV1

decrement). However, because there are 

still some ambient concentration increases 

that occur in the 50 to 60 ppb range where 

the estimated lung function risk model is 

more responsive, there may be some 

reduction in the net risk decrease when 

adjusting air quality to just meet 

progressively lower standard levels. 

The mortality and morbidity risk assessment is 

the analysis that is most sensitive to the 

increases in O3 in the lower part of the O3

concentration distribution that we estimated 

would occur when adjusting air quality to 

just meet the existing and alternative 

standards some urban study areas. Mean O3

concentrations in the urban study areas 

change little between air quality scenarios 

of just meeting the existing standard and 

progressively lower alternative standard 

levels, because mean concentrations 

reflect both the increases in O3 at lower 

concentrations and the decreases in O3

occurring on days with high O3

concentrations. This leads to small net 

changes in mortality and morbidity risk 

estimates for many of the urban study areas. 

However, both the net change in risk and 

the distribution of risk across the range of O3

concentrations may be relevant in 

considering the degree of additional 

protection provided by just meeting existing 

and alternative standards. 

In conclusion, we have estimated that 

exposures and risks remain after just meeting 

the existing O3 standard and that that in 

many cases, just meeting potential 

alternative standard levels results in 

reductions in those exposures and risks. 

Meeting potential alternative standards has 

larger impacts on metrics that are not 

sensitive to changes in lower O3

concentrations. When meeting the 70, 65, 

and 60 ppb alternative standards, the 

percent of children experiencing exposures 

above the 60 ppb health benchmark falls to 

less than 20 percent, less than 10 percent, 

and less than 3 percent in the worst O3 year 

for all 15 case study urban areas, 

respectively. Lung function risk also drops 

considerably as lower standards are met. 

When meeting the 70, 65, and 60 ppb 

alternative standards, the percent of 

children with lung function decrements 

greater than or equal to 10 percent in the 

worst year falls to less than 21 percent, less 

than 18 percent, and less than 14 percent in 

the worst O3 year for all 15 case study urban 

areas, respectively. Mortality and 

respiratory hospitalization risks decrease by 

5% or less for a level of 70 ppb, 10% or less for 

a level of 65 ppb, and 15% or less for a level 

of 60 ppb. These smaller changes in the 

mortality and morbidity risks, relative to the 

exposures and lung function risk reductions, 

reflect the impact of increasing O3 on low 

concentration days, and the non-threshold 

nature of the C-R function. Larger mortality 

and morbidity risk reductions are estimated 

on days with higher baseline O3

concentrations. 

While there remain significant uncertainties 

identified in each of the analytical areas, 

we have sufficient confidence in the overall 

results for them to be useful in informing the 

policy assessment. Our assessment suggests 

that the highest confidence should be 

placed in the results of the human exposure 

and lung function risk results, largely 

because they are based on results of 

controlled human exposure studies and a 

physiology-based risk model. Medium to 

high confidence is placed in the results of 

the assessment of epidemiology-based risks 

associated with short-term O3 exposures, 

because while the large number of studies 

supporting the C-R relationships provides 

increased confidence, there still exists 

uncertainties related to unexplained 

heterogeneity between locations, exposure 

measurement errors, and interpretation of 

the shape of the C-R function at lower O3

concentrations. Lower confidence is 

placed in the results of the assessment of 

epidemiology-based mortality risks 

associated with longer-term O3 exposures, 

primarily because that analysis is based on 
 

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Executive Summary Health Risk and Exposure Assessment 

for Ozone, Final (July 2014)

only one well designed study, and because 

of the uncertainty in that study about the 

existence and location of a potential 

threshold in the C-R function. 

References 

Henderson, R. (2006). Clean Air Scientific 

Advisory Committee’s (CASAC) Peer Review 

of the Agency’s 2nd Draft Ozone Staff 

Paper. U.S. Environmental Protection 

Agency Science Advisory Board. EPACASAC-07-001. 

Simon, H.; K. R. Baker; F. Akhtar; S.L. 

Napelenok; N. Possiel; B. Wells and B. Timin. 

(2013). A direct sensitivity approach to 

predict hourly ozone resulting from 

compliance with the national ambient air 

quality standard. Environmental Science 

and Technology. (47):2304-2313. 

U.S. EPA. (2012). Integrated Science 

Assessment of Ozone and Related 

Photochemical Oxidants (Final Report). 

Research Triangle Park, NC: EPA Office of 

Research and Development. (EPA 

document number EPA/600/R-10/076F). 

<http://cfpub.epa.gov/ncea/isa/recordispl

ay.cfm?deid=247492#Download>. 
 

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United States 

Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Health and Environmental Impacts Division 

Research Triangle Park, NC 

Publication N